The main characteristic of human myopia is axial elongation, and the main part of axial elongation is the posterior part of eyeball. Previous studies have found that a normal human eye is significantly different from a myopic eye in collagen bundle structure, fiber diameter distribution, fiber configuration, and so on. Now, it is found that phenomena, such as thinning of the sclera, thinning of the scleral collagen fibrils, and disappearance of the gradient changes in the inner, middle and outer layers of the fiber diameters of the sclera, appear in the long-time experimental myopic eyes of mammals (e.g., tree shrew, marmoset, and guinea pig). These findings indicate that in the developmental process of myopia, the changes in the sclera structure of an myopic eye involve a positive remodeling process.
In recent years, researches on mammal myopic models (e.g., in monkey and tree shrew) showed that, in the experimental myopic eyes, growth of fibroblast is inhibited, collagen synthesis is reduced, and dry weight of sclera is reduced. Biomechanics studies also revealed that myopia resulted in a weak sclera and caused the changes in sclera stress. It is detected that the creep rate of the sclera of a myopic eye increased, the elasticity of the sclera increased, and the failure load of the tissue decreased. The above mentioned changes mainly appear in the posterior pole of the sclera. The biological and chemical changes of the sclera resulted in changes of physical and mechanical characteristics of the sclera, and eventually resulted in the development of myopia.
In conclusion, sclera is regarded as the target tissue for the occurrence and development of myopia. Hence, regulating the activation and differentiation of sclera fibroblasts may influence the occurrence and development of myopia. Kohyama T, et al. (2002) and Liu X, et al. (2004) found that regulating the intracellular cAMP (cyclic adenosine monophosphate) level may influence the activity of lung fibroblast. cAMP plays a role as a second messenger in the reaction of inhibiting the activity of fibroblasts. Differentiations of lung and cardiac fibroblasts can be inhibited by increasing the intracellular cAMP levels; conversely, activation and differentiation of lung fibroblasts can be promoted by decreasing the cAMP levels. However, at present there is no report about the effects of cAMP on sclera fibroblasts, in particular the effects of cAMP on myopia.